Antimicrobial properties of allium species

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The antimicrobial activity of Allium species has long been recognized, with allicin, other thiosulfinates, and their transformation products having antimicrobial activity. Alliums are inhibitory against all tested microorganisms such as bacteria, fungi, viruses, and parasites. Alliums inhibit multi-drug-resistant microorganisms and often work synergistically with common antimicrobials. Allium-derived antimicrobial compounds inhibit microorganisms by reacting with the sulfhydryl (SH) groups of cellular proteins. It used to be thought that allicin reacts only with cysteine and not with non-SH amino acids, but evidence has accumulated that allicin and other thiosulfinates also react with non-SH amino acids.

Highlights

► Antimicrobial principles of fresh alliums are thiosulfinates represented by allicin. ► Several different antimicrobial compounds are also formed in heated alliums. ► Alliums inhibit all microorganisms including bacteria, fungi, viruses and parasites. ► Allium antimicrobials are known to react with SH groups of cellular proteins. ► Evidences that thiosulfinates react with non-SH amino acids are accumulating.

Introduction

The principal antimicrobial compounds of fresh alliums are newly formed by degrading S-alk(en)yl-l-cysteine sulfoxides (sulfoxides), noninhibitory sulfur-containing secondary metabolites, by the action of alliinase after their tissues are damaged [1]. The sulfur-containing antimicrobial compounds derived from sulfoxides in alliums are not found naturally, unlike some other antimicrobial secondary metabolites in plants such as phenolics, terpenoids, alkaloids, organic acids, and polypeptides [2].

The antimicrobial activity of alliums is mainly attributed to various kinds of alk(en)yl alka/ene thiosulfinates (thiosulfinates; [3]) and their transformation products [3, 4, 5, 6, 7], with minor contributions from peptides [8], flavonoids [9], phenols [9], alkaloids [10], and saponins [11]. In this review, only the antimicrobial activity of sulfoxides degradation products is considered. The reactivity of thiosulfinates with non-SH amino compounds is also addressed in relation to the instability of allicin in homogenized garlic and in complex food systems.

Section snippets

Allium species and their antimicrobial precursor compounds

Various Allium species have received attention for their antimicrobial activity, including garlic (A. sativum L.) [7, 12, 13, 14], onion (A. cepa L.) [12, 14], shallot (A. ascalonicum L.) [15•, 16], elephant garlic (A. ampeloprasum L. var. ampeloprasum auct.) [17], and wild garlic (ramsons; A. ursinum) [18]. The antimicrobial activities of allium extracts of bulbs [7, 12, 13, 16] and of essential oils [14, 15•, 17, 19] have been investigated. Allium leaves and flowers have also been examined

Antimicrobial compounds and their mechanism of action

Allium antimicrobial compounds differ depending on processing (Figure 1). When freshly crushed, various thiosulfinates [3] are present, when crushed and stored, various dialk(en)yl sulfides are revealed, when macerated in oil, ajoene is present, and when heated at 121°C, heterocyclic sulfur compounds [5], allyl alcohol [6], and 3-(allyltrisulfanyl)-2-aminopropanoic acid [7] occur. Steam-distilled oils of crushed Allium contain various sulfides distinctive to each species.

Thiosulfinates

Antimicrobial activity of Allium species

Allium species have antimicrobial activity against bacteria, fungi, viruses, and parasites. Most research has focused on the antimicrobial activity of garlic followed by onion. However, intermittent reports on other Allium species have appeared.

The antibacterial efficacy of allium is slightly different depending on the extraction solvents used. Water [18, 28], ethyl acetate [18], and ethanol [29, 30] are more frequently used compared with other solvents including acetone [18], chloroform [18, 30

Applying alliums as alternative food preservatives

Alliums have been tested as direct natural food preservatives, but the results are conflicting. Some researchers claimed significant protective effects [15•, 17, 44], whereas others observed no or only insignificant effects [14, 45, 46].

The inhibition of microorganisms by alliums and allium-derived products is apparent when they are tested in laboratory media, but not in actual complex foods. Garlic (5%) in buffered peptone water decreases Salmonella typhimurium, whereas the same amount of

Conclusions

Allicin and other alliin-derived products have significant inhibitory activity against all microorganisms including bacteria, fungi, virus, and parasites when tested in laboratory media. Allicin is significantly less stable in homogenized garlic and in complex food systems than in a pure state in water because allicin and thiosulfinates, in general, react with amino acids and other amino compounds in garlic and in foods. Not only odor and bad breath but also the reactivity of thiosulfinates

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

This work was supported by the Technology Development Program of Ministry of Food, Agriculture, Forestry, and Fisheries, Republic of Korea. The assistance of Mr. TY Oh with the figures is greatly appreciated.

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